The present invention relates to a meltable ink composition, for use in printing devices in which ink drops are ejected from ink ducts. Inks of this kind, which are also known as hot melt inks or phase change inks, are solid at room temperature and liquid at elevated temperatures. In a printing device, for example, an ink jet printer, the ink is melted and transferred to substantially closed ink ducts which lead at one end into a nozzle. As a result of a sudden pressure rise in the duct, produced for example by rapidly reducing the volume of the duct or rapidly evaporating some of the ink in the duct, a drop of ink can be ejected from the duct through the nozzle. In this way an image can be built up from individual ink drops on a receiving material. Meltable inks contain a carrier material with a melting point higher than room temperature so that the ink melted in the printing device is practically not subject to evaporation when the printing device is not printing. This prevents the nozzles of the ink ducts from becoming clogged. In addition, inks of this kind have the advantage that they set rapidly on the receiving material, so that the print quality, particularly the edge sharpness, is less dependent on the type of receiving material.
U.S. Pat. No. 4,851,045 discloses oxidized montan waxes which are the reaction product of aliphatic diols and C25-35 fatty acids. These waxes have the disadvantage that their melting temperature is relatively low, so that printed ink images may result in blocking at elevated temperature or bleeding through the paper. In addition, the crystallized waxes are relatively soft, so that printed images may feel xe2x80x9cwaxyxe2x80x9d and be less resistant to mechanical stresses. Finally, the viscosity of these esterified montan waxes is relatively high, so that it is difficult to use them in ink compositions for ink jet printers.
U.S. Pat. No. 4,931,095 discloses benzoates as a component of meltable inks. Compounds of this kind are formed by esterifying benzoic acid, an unsubstituted aromatic acid, with a suitable alcohol. From the patent specification it is known to use polyalcohols, such as pentaerythritrol, sucrose and glycerol for this purpose. These benzoates have the disadvantage that they set amorphously when they cool rapidly from a melted state, particularly when a liquid ink drop is transferred to a relatively cold receiving material. As a result, the solidified ink drops are to some extent tacky at room temperature, particularly when mechanically stressed. By gumming, scratching or rubbing over a printed image, for example, the ink may block. In addition, the compounds may subsequently crystallize after being transferred to a receiving material, so that quality of the image, particularly the visual quality, deteriorates in the course of time.
EP 0 856 565 A1 discloses the toluate and anisylate esters of pentaerythritol as a component of a meltable ink. These compounds differ from the benzoates due to the presence of a substituent on the aromatic benzene ring. As a result, the physical and chemical properties of these compounds differ from those of the benzoates. These toluates and anisolates, however, also have the disadvantage that they solidify amorphously when cooled rapidly. In addition, the compounds have a relatively intensive after-crystallization. They also have the disadvantage that their melting temperature is relatively high, thus making them less suitable as a component for a meltable ink. A higher melting temperature makes much higher demands of the printing device itself.
The toluate and anisylate esters of 2,2xe2x80x2-bisphenol are known from EP 0 978 548 A1. These substances result in inks which exhibit a deterioration in copy quality if the prints are stored for a long time, for example some months. This is probably the result of a slow after-crystallization from an amorphous state.
A specific 1,4-bis-[(substituted)-benzoyloxymethyl]-cyclohexane is known from JP 2076786 as a sensitivity improver for a color developer in a sheet suitable for thermal transfer. This patent does not indicate or suggest that these compounds can also be used in meltable inks suitable for ink jet printers. The latter inks are subject to totally different requirements from inks for thermal transfer. Ink jet printers differ from thermal printers in that the liquid ink in the form of individual ink drops ejected through air over a certain distance, is transferred to a receiving material. As already indicated hereinbefore, the print heads used for this purpose have ink ducts provided with nozzles and means for ejecting one ink drop at a time from a specific nozzle. To achieve this drop formation, the physical properties of the ink, particularly the viscosity and surface tension, must lie within narrow limits. In addition, meltable ink is frequently heated a number of times in an ink jet printer and then cooled again to room temperature before actually being transferred to a receiving material. This places high demands on the thermal stability of the ink. All this makes the development of meltable inks for ink jet printers a particularly complex area.
High-boiling organic solvents for use in a silver halide photographic material are known from U.S. Pat. No. 4,923,783, in accordance with formula I
(ArCOO)nxe2x80x94L xe2x80x83xe2x80x83(I) 
wherein Ar is an aryl group with 6 to 24 carbon atoms, L is a 2-valent to 6-valent alkylene with 2 to 24 carbon atoms possibly containing one or more ether bonds and n is an integer between 2 and 6. Compounds of this kind reduce the reductive bleaching of cyan dyes as a result of damp and heat. This patent contains no suggestion that these substances can be used as a component in a meltable ink.
An object of the present invention is to provide a meltable ink composition having advantageous properties for use in printing devices in which the ink is ejected in the form of drops from ink ducts. Another object of the present invention is to provide ink compositions which result in printed images having a good and stable print quality. A further object of the present invention is to provide inexpensive ink compositions. To this end, an ink composition has been discovered which contains a compound in accordance with the following formula II 
where R1 and Rxe2x80x21 are the same or different and are selected from a C1-8 alkyl group or a C1-8 alkoxy group, R2 is a C1-12 linear alkane diyl or a C5-12 alkane diyl which contains an alicyclic group, and n is 1, 2, or 3. These bis-ester compounds have melting temperatures suitable for forming a meltable ink composition usable in an ink jet printer. The melt viscosity of the compounds is also sufficiently low, typically lower than 20 cP, at the regular jet temperatures. The compounds are also such that they can result in inks sufficiently resistant to mechanical stress. Finally, these compounds are stable in the solidified state, so that the risk of deterioration of the copy quality of printed images is reduced. Compounds of this kind can advantageously be used in meltable ink compositions. In this way the present invention counters the disadvantages of the prior art ink compositions as described above.
Advantageously, R1 and Rxe2x80x21 are selected from a C1-6 alkyl group or a C1-6 alkoxy group. With these compounds it is possible to obtain ink compositions which are less waxy. With ink compositions of this kind, therefore, it is possible to obtain images which are more resistant to mechanical stress such as rubbing, scratching, gumming or folding. More preferably, R1 and Rxe2x80x21 are selected from a C1-4 alkyl group or a C1-4 alkoxy group. These compounds are even less waxy and can thus result in ink compositions for printing images which are even more resistant to mechanical stresses.
In one preferred embodiment, R1 is equal to Rxe2x80x21. In this way, better-defined compounds can be obtained, the properties of which can be more satisfactorily controlled. In this way it is possible to obtain an ink composition which is better adapted to the specific requirements that a specific application imposes for the ink. In addition, a better defined product of this kind can be obtained in a purer form which enables the reliability of use of the ink composition to be increased.
Preferably, R1 and Rxe2x80x21 are a C1-4 alkoxy group. These compounds crystallize more rapidly. Ink compositions provided with such compounds can result in printed images which are resistant to mechanical stresses very soon after actual printing. In this way it is possible to subject printed receiving material to further processing practically immediately, for example in a finishing station in order to make up sets of documents. In addition, these ink compositions have the advantage that they are even more stable.
In one preferred embodiment R1 and Rxe2x80x21 are a C1-2 alkoxy group. Compounds of this kind are simpler and hence less expensive to produce, this being advantageous in terms of the cost of the ink composition. Even more preferable, R1 and Rxe2x80x21 are a methoxy group. Very good ink compositions according to the present invention can be obtained with these compounds.
In one embodiment n is equal to 1. The resulting compounds can easily be obtained in pure form, resulting in a good ink composition.
In a further embodiment, R1 and Rxe2x80x21 are substituted at the para position. It has been found that compounds of this kind can crystallize relatively quickly and are stable in the solidified state. The reason for this is not entirely clear, but is probably due to the symmetry in the molecule, so that it is easier to build up a crystal lattice. Ink compositions which contain such compounds become hard relatively quickly when transferred to a receiving material and are stable in the course of time. A high print quality can be obtained in this way.
In one specific embodiment, R2 is a C2-C6 linear alkane diyl. These compounds have a very favorable melting point and are relatively hard in the crystallized state. With these it is possible to obtain ink compositions which are improved with respect to resistance to mechanical stress.
In a further embodiment, the ink composition contains a compound selected from the group consisting of bis-esters of paramethoxy benzoic acid with 1,3-propane diol and 1,6-hexane diol. With these compounds it is possible to obtain very good ink compositions according to the present invention. In another embodiment, R2 is a C6-C8 alkane diyl containing an alicyclic group. These compounds have a higher melting point so that an ink composition can be obtained which is very resistant to mechanical and thermal stresses.
In a preferred embodiment in which the alicyclic group is substituted so as to have a cis and a trans-conformation (one example of such an R2 group is 1,4-cyclohexane dimethylene), the ink composition contains the trans-conformation of the alkane diyl. This stereoisomer crystallizes more satisfactorily than the cis-conformation so that an ink composition can be obtained with better mechanical properties. In addition, this compound results in ink compositions which are more stable in time, because after-crystallization will occur to a lesser degree.
In yet a further preferred embodiment, the ink composition substantially contains the trans-conformation of the alkane diyl. Inks with further improved properties can be obtained in this way.
Preferably, the ink composition contains 10-99% by weight of compounds according to formula II. It has been found that the formula II compounds are particularly suitable as a carrier material for a meltable ink composition, because in this way it is possible to form ink compositions with an optimal melting temperature and melt viscosity. Also, the ink compositions, after cooling, are very resistant to mechanical stress and are stable over the course of time. It has been found that the compounds according to formula II can advantageously be used in a meltable ink provided with a gelling agent.
In a further embodiment, the ink composition contains 30-99% by weight of one or more formula II compounds. In this way the advantages of this compound can be further utilized so that meltable inks can be obtained with even more favorable properties.
The compounds according to formula II can be used in many ways in meltable inks suitable for use in ink jet printers. For example, they can be used in relatively large quantities as a carrier material in an ink composition and, for example, in smaller quantities as a viscosity regulator, softener, mixture improver, and so on. In addition to a single compound, ink compositions according to the present invention may also make use of mixtures of these compounds. In addition, these compounds can be used in combination with known hot melt ink components, such as waxes, paraffins, resins, softeners, viscosity regulators and so on. The final composition depends on the specific requirements the ink must meet, and this depends on the type of ink jet printer, the required print quality, the type of receiving material, the specific application, and so on.
The present invention will now be explained further with reference to the following examples and tables which are given as examples only and are not limitive of the present invention.
Example 1 shows a number of ink compositions according to the present invention. Comparative Example 2 gives a number of ink compositions containing compounds which are known in the prior art. Example 3 indicates how formula II compounds can be made.
Table 3 gives a number of examples of formula II compounds.
Table 4 shows a number of compounds known from the prior art.